In Mississippi we say "done for" when we mean "done for good". Maybeso the unspoken "good" adds finality as does "done for sure".And maybeso "done for God" was once a religious phrase used at funerals which later transmuted into "done for good". But "done for good" is semantically illogical so we say "done for" with the "good" serving as an unspoken intensifier of "done".

My interpretation of the phrase, with absolutely no authoritative backup, is that it relates to "doing for" someone in the sense of domestic service, as a valet or a lady's maid: "Robert was hired to do for Mr. Halt-Rodney." When one is "done for," all possible service has been rendered, further service is neither necessary nor possible.

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